1. Field of Invention
This invention relates to a microwave moisture sensor which operates on the principle that an emitted microwave is attenuated in rotary resonance with water molecules, and more particularly to a microwave moisture sensor comprising a microwave transmitter and a microwave receiver spaced a distance from each other in confronting relation for producing a signal, indicative of the water content in a sheet of material to be measured, which travels between the microwave transmitter and receiver.
2. Description of Prior Art
Microwave moisture sensors are known in the art. One example of such a prior sensor is depicted in FIG. 1 and comprises a detector head 30 having metal casings 1,2 housing a microwave transmitter and a microwave receiver. The detector head 30 is mounted on a centrally open frame 31, as shown in FIG. 2, for reciprocally scanning a sheet of paper 23 in a transverse direction thereof, across a zone between limits L1 and L2, to thereby measure the moisture or water content in sheet of paper 23, which travels in the direction of the arrow A, through frame 31. Casing 1 accommodates the microwave transmitter and receiver, generally designated by 6 and 10, respectively. Microwave transmitter 6 includes a microwave oscillator 3, an isolator 4, and a microwave transmitting horn antenna 5. Microwave receiver 10 includes a microwave receiving horn antenna 7, a detector (e.g. Schottky diode) 8, and a signal amplifier 9 which amplifies a detected signal and supplies the amplified signal to a signal processor (not shown). The signal processor is supplied with the signal from detector head 8 and a preset signal, representative of the basis weight of paper 23, and processes the supplied signals based on a predetermined calibration curve. Detector 8 comprises a heterodyne detector energized by a local oscillator (not shown) which generates a signal having a frequency different from that of the signal produced by microwave oscillator 3. Casing 2 has a microwave receiving horn antenna 11 which cooperates with antenna 5 and together can be considered to constitute a transmitting and receiving pair. Also in casing 2 is a microwave transmitting horn antenna 12 which cooperates with antenna 7 and together can be considered to constitute another transmitting and receiving pair. Antenna 12 is connected to antenna 11 by a coaxial cable 13 for transmitting the microwave that has been received by antenna 11. Casings 1,2 have microwave transmitting and receiving ports closed by thin films 18,19, 20,21, which films may be of such a material as polyethylene terephthalate.
For moisture measurement, the microwave moisture sensor moves detector head 30 to scan the sheet of paper 23, transversely across the zone L1-L2 (see FIG. 2). At this time, a microwave emitted from transmitter 6 to paper 23 travels from antenna 5 through space 14 and film 18 to paper 23, then through film 20 and space 16 to antenna 11, then to coaxial cable 13, then to antenna 12, then through space 17 and film 21 to paper 23 and then through film 19 and space 15 to antenna 7, Then, the microwave is detected by detector 8. The signal processor (not shown) is responsive to the detected signal, as amplified by amplifier 9, and to the preset signal, indicative of the basis weight of the paper 23, for processing these signals based cn the calibration curve, to thereby generate a signal representative of the water content or moisture in paper 23.
The prior microwave moisture sensor just described is disadvantageous inthat its measurement accuracy is low since the moisture dependent signal (input signal) fed to the signal processor contains drifts of the microwave oscillator, the detector and other devices.
Another problem with the conventional microwave moisture sensor is that with the basis weight signal applied to the signal processor being preset, the measurement accuracy is lowered when the basis weight of the paper is changed during measurement. It should be noted that the basis weight of paper of one brand or manufacturer may vary in the actual paper making plant.
In addition, the measurement accuracy is also lowered when the paper is subjected to temperature variations because the degree to which the paper absorbs microwaves, varies with the temperature.